Recently there has been much interest in providing data storage devices containing flash memory and which can be connected to a serial bus of a computer. Such devices are used for data storage, data transfer, media players such as, for example, MP3 players, and so forth. The structure and operation of such a portable storage device may be as shown in the earlier patent. Tradition flash memory is the SLC flash memory. The newer, MLC flash memory has a read and write speed that is much slower than SLC flash memory. Many devices are now including MLC flash memory but the operation of such devices is slower than if only SLC memory was used.